The present invention relates to an apparatus for controlling and regulating a movement of a system including a plurality of individual elements which cooperate kinematically, at least one thereof being a drive. The present invention further relates to a method for controlling and regulating a movement of such a system. Such a system is understood, for example, as equipment, a machining tool, a processing machine and, in particular, also a robot or a machine tool. The drive is, for example, an electric motor or a hydraulic or pneumatic drive.
The following discussion of related art is provided to assist the reader in understanding the advantages of the invention, and is not to be construed as an admission that this related art is prior art to this invention.
Mechanical systems of this type are widely used in manufacturing. An important prerequisite for achieving good results in terms of quality is the absolute precision of these manufacturing means. However, external interfering forces (for example gravitational force, process force, friction, load change, etc.) act on the mechanical structures, said forces potentially leading to deviations at the so-called “TCP” (Tool Center Point) due to the resilience of the joints and components (hereinafter also called individual elements). The positional dependency of the deviation at the TCP is in some circumstances very high due to the interfering forces. Thus, for example, a robot arm in the extended state is pulled further down at the free end than in the retracted or angled state. The absolute static precision is a measurement thereof.
Hitherto, in the case of controlled axes, this problem of absolute static precision has been resolved by a second direct measurement system being used on the load side in the vicinity of the TCP. The position of the motor and/or drive is adapted via the position control circuit to a sufficient extent that the required compensation force is transmitted to the structure via the drive train suspension. The use of a second measurement system is associated with considerable additional hardware costs and requires a complex adaptation of the structure in order to install the measurement system.
A further approach for improving the absolute static precision is to measure, relative to a reference position, the deviation at the TCP in a plurality of points of the working area and to add this value as a compensation value to the reference value channel and/or to subtract this value from the actual value channel. The drawback with this method is the effort and costs required for the measurement of the working area and/or of the robot/machine. The measurement of the deviations requires the equipment to be provided with external measuring means. The measuring pattern is established on a case-by-case basis. The analysis has to be repeated in each piece of equipment of the same construction. The correction values stored in tabular form are additionally load-dependent, i.e. they apply specifically to one loading case. If the loading changes, for example, the values no longer apply.
To this end, a robot control is disclosed in the publication EP 1 980 374 A2 in which an absolutely precise model is stored for controlling an industrial robot. At the start point and end point of the movement, therefore, interfering forces for the positioning are taken into account in the respective steady-state system.
A further important feature is the so-called “quasi-static precision”. During the acceleration phase the motor force and counter force act equally in one joint. The counter force is based on a structure which is not infinitely rigid and may in any case cause parasitic movements at the TCP. If a robot arm, for example, has a plurality of members which in each case are able to be moved relative to one another by means of a motor (a member with an associated drive and/or motor represents an axis of movement, abbreviated to “axis”) the movement, in particular an acceleration, of the one member may have an effect on the other member, whereby the quasi-static precision is affected. To this end, the parasitic movement produced in machine tools due to an acceleration of the control axis has hitherto been compensated by a compensating value applied as a reference position value.
During the rotational movement of an axis, the torque is frequently controlled by so-called torque pilot control. The parameterization of the torque pilot control requires the input of the axial moment of inertia relative to the motor. However, in industrial robots this value is not constant over the working area, due to the construction. The total inertia relative to the motor is a function of the joint position. To calculate a reference torque value the motor acceleration has hitherto been multiplied by a predetermined value of the axial inertia.
The parameterization of the maximum axial acceleration is carried out as a result of the design of the motor and the power output component and is derived relative to the axis from the maximum permitted current and the axial inertia. As the maximum acceleration has to be recorded as a constant value, for the parameterization thereof only the point where the axial inertia relative to the motor reaches its maximum is relevant. As a result, in the regions where this value is not reached, the current reserve is not utilized and a higher acceleration limit would theoretically be possible. Therefore, in some circumstances it could be desirable to calculate an acceleration reserve for each position and/or an adaptation of the acceleration limit for greater exploitation of the current limit. Position-dependent acceleration limits have hitherto not been considered in the path planning.
In particular in industrial robots, the mobility and flexibility of the operating means is highly valued. However, this requires structural boundary conditions which make the robot into a structure which is highly susceptible to vibrations. Due to the absence of measuring options an electrical damping of these vibrations is not possible. A portion of the excitation results directly from the command variables. Therefore the command behavior should be correspondingly adapted according to the vibrations. One possible remedy is to moderate the frequency content of the travel profiles by reducing the jerk so that natural resonances are no longer excited. The drawback with this method is the necessary restriction of the axial dynamics which can be considerable.
Due to the movement transformation which can be complex and the resulting compensation movements, it is no small task to identify the risk of collisions in advance. Monitoring the measurement signals (for example the motor current) is also not possible due to the loading which can be highly variable. Thus improved monitoring of collisions might be desirable. In hitherto known solutions to this problem, the motor currents calculated by means of a model are compared with the actually measured motor currents, and when the deviations are inadmissibly high the machine/robot is stopped. Such a system is disclosed, for example, in the publication EP 1 403 746 B1.
An apparatus and a method for controlling and regulating a movement of a system comprising a plurality of individual elements which cooperate kinematically, at least one thereof being able to be moved by a drive, is disclosed in the publication DE 10 2007 024 143 A1. Force values identifying the robot are continuously calculated during the movement of the system. Moreover, a compensatory compensating variable based on reference coordinates is also continuously calculated during the movement of the system. A force-producing variable for the at least one drive is controlled continuously during the movement depending on the reference coordinates and the at least one compensating variable.
It would therefore be desirable and advantageous to provide an apparatus for controlling and regulating and a corresponding method for controlling and regulating a movement of a system, comprising a plurality of individual elements which cooperate kinematically, in which the movement of the system per se may be performed more precisely.